1. Field of the Invention
The present invention relates generally to audio amplifiers, and more specifically to a distortion limiter to minimize amplifier clipping.
2. Related Art
Typically audio reproduction systems include a variable-gain preamplification stage, followed by a fixed-gain power amplifier. The power amplifier drives an output transducer such as a speaker. A volume control voltage, provided to the preamplification stage, controls the signal level provided to the power amplifier stage and thus controls the output volume.
One objective in designing audio systems such as the one described above is to introduce a minimum amount of distortion into the audio signal. Typically, as the magnitude of the signal provided from the variable-gain preamplification stage to the power amplifier increases above a certain level, the power amplifier may be overdriven. This occurs when the input signal to the power amplifier, multiplied by the fixed gain of the power amplifier, approaches the supply voltage provided to the power amplifier. At these high input signal levels, the power amplifier saturates and the signal peaks of the audio signal are distorted. This phenomenon is known as clipping.
In some applications, such as portable systems or automotive audio systems, the problem of clipping is more severe. This is because these systems have a limited electrical supply voltage (e.g. the 12-volt automotive system) which results in limited voltage headroom provided to the amplifier.
Many conventional distortion limiters use a feedback loop to limit the level of the input signal. In one system, described in U.S. Pat. No. 4,048,573, to Evans, et al. (referred to as '573), a difference amplifier 30 is used to amplify the difference in signal level between input and output signals of a power amplifier 20. The output signal of power amplifier 20 is provided to difference amplifier 30 via a feedback circuit 25. The output signal of difference amplifier 20 is fed to a full wave rectifier 32, the output of which drives an attenuator 31. When an excess signal is input to power amplifier 20 causing clipping, differential amplifier 30 outputs a signal in accordance with the difference between the amplifier input signal and the amplifier output signal. This difference results in an attenuation of the input signal by way of rectifier 32 and attenuator 31.
Another disadvantage with the '573 reference is that the feedback signal is provided to a light-emitting diode to control the amount of signal shunted to ground via a light-dependent resistor (in attenuator 31). Such optical devices are relatively costly, may be unreliable and provide a relatively slow response time to the occurrence of clipping.
A further disadvantage of the '573 reference is the manner in which differential amplifier 30 is connected to power amplifier 20. The inputs to amplifier 20 are also the inputs to amplifier 30. Thus the inputs to amplifier 20 are common mode voltage on amplifier 30. Because there is potentially a large voltage swing on differential amplifier 30, additional noise in the circuit could be introduced. Additionally, there are two connections between the two amplifiers. As a result, the common mode range is more critical due to loading.
Another conventional technique is disclosed in U.S. Pat. No. 4,581,589, to Ikoma (referred to as '589). According to the '589 reference a reference means 41 provides a first reference voltage lower than the upper saturation voltage (upper clipping level) of the amplifier 20, and a second reference voltage higher than the lower saturation voltage of amplifier 20. These reference voltages are provided to a comparing and controlling means 40 for comparison with the output signal from amplifier 20. When the output signal goes out of the range defined by the saturation voltages (the reference voltages), comparing and controlling means 40 produces an output signal that is fed to the signal attenuator to reduce the input signal.